Description:

Recent observations have suggested that gas phase chemical reaction may play a role in the atmospheric transformation of mercury. The overall goals of the proposed research program are a series of measurements and technique developments that will allow both the chemical reactivity, the atmospheric concentrations, and the rates of emission and deposition of both elemental and reactive gaseous mercury, to be better defined. Laboratory studies will measure the rate coefficients for the reactions of Hg(0) with the hydroxyl radical and with halogen atoms, X, and halogen monoxides, XO , where X= Cl, Br, I. When feasible the reaction products and their yields will be identified. Reactions will be studied under conditions that are representative of the arctic, the upper troposphere and the global marine boundary layer. In addition we will investigate the feasibility of laser based excitation schemes for the rapid, ultrasensitive detection of gas phase elemental mercury and reactive gaseous mercury.

Approach:

The proposal builds on a number of experimental capabilities in chemical kinetics and gas phase ultra-sensitive detection that have been demonstrated in this laboratory. Kinetic studies will utilize laser photolysis for radical generation, coupled with simultaneous detection of both Hg(0), the free radical of interest, and were possible the reaction products, using laser induced fluorescence (LIF). Temporal profiles will be obtained by monitoring the concentrations as a function of delay time between the photolysis and detection lasers. Kinetic studies will be performed as a function of temperature, pressure and gas composition. For mercury this requires measurements in air at pressures of up to one atmosphere over the temperature range 300-240 K. This will be accomplished using a novel kinetic scheme in which the pseudo-first order decay of mercury is monitored in an excess of the free radical of interest.

Expected Results:

The rate coefficients for the reactions of Hg(0) will be used in conjunction with atmospheric models to assess the role of the various halogen species in the rapid depletion of Hg(0) in the arctic depletion events which occur in the arctic spring. The results will also be used to assess the rate of global chemical conversion of Hg(0) to chemically bound Hg in the marine boundary layer, via halogen chemistry, and in the upper troposphere, by reaction with OH. The identities of the reaction products and their yields will be used to predict deposition rates. In addition they identify the species which should be monitored in field experiments to test atmospheric models. A significant increase in time response and sensitivity of Hg(0) measurements will allow the technique to be used for real-time eddy-correlation measurements of mercury deposition. A detailed understanding of the rates and mechanism of mercury deposition is a critical component of any model of atmospheric mercury transformation. Extension of laser based detection techniques, increasing the sensitivity of total or speciated reactive gaseous mercury detection, would allow improved measurements of atmospheric concentrations and the factors which control their variability such as atmospheric formation and deposition rates.

Progress and Final Reports:

The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.